The present invention relates to an optical system for scanning an object surface with a beam of light originating at a light source, and, more particularly, to such an optical system particularly suited for use in laser printers.
An optical system used in laser printers may be characterized as having three sub-systems, namely, a pre-scan optical sub-system, a scanning sub-system, and a post-scan sub-system. Typically, the pre-scan optical sub-system includes a laser diode with large beam divergence for a light source, a collimator lens to produce a collimated beam of the light emitted by the laser diode, a pre-scan lens to focus the process beam to a waist, a plane pre-scan mirror to fold the pre-scan optics path and to attenuate laser power, and associated mounting hardware.
The scanning sub-system is essentially a motor driven rotatable polygonal reflector having adjacent peripheral mirror surfaces or facets that both translate and rotate in operation of the printer. The mirror surfaces reflect the collimated and focused beam of the pre-scan optical sub-system and their direction of translation determines the scan direction of the beam passing to a scanned object, that is, a photosensitive drum in a laser printer.
The post-scan optical system conventionally includes a focusing lens for transforming the light beam reflected from the polygonal reflector of the scanning sub-system into a beam having spot size suitable for the laser printing operation, and a lens known in the art as an f-theta lens. The f-theta lens functions principally to compensate spot positional dependence on the tangent of the scanning mirror rotation angle .theta. in order to produce a nearly linear position to angle relationship. The post-scan optical sub-system may also include one or more folding mirrors to adapt to the geometry of the printer apparatus.
The pre-scan optical sub-system defines the light beam axis between the laser diode source and the rotatable polygonal reflector and provides a set of beam diameters and radii of curvature at two locations on that axis. Although the optical components used in this sub-system are relatively uncomplicated from a design standpoint, the pre-scan optical sub-system utilizes very short focal length optics of high numeric aperture for reasons of size and efficiency of coupling to the laser diode. As a result, sensitivity to component tolerance and placement accuracy are very important. Also, the pre-scan optical sub-system is required to produce a beam waist in the cross scan or processing direction, perpendicular to the scan direction, at a precise location relative to the polygonal reflector. This requirement has been satisfied in the prior art by a plano-cylindrical lens oriented with the axis of the cylinder parallel to the scan direction.
The polygonal reflector of the scanning sub-system of laser printers rotates at speeds in the range of tens of thousands of revolutions per minute and, as such, represents a source of audible and optical noise that has a deleterious effect on the ultimate quality of laser printer performance. Therefore, reduction in size and inertia of the rotatable polygonal mirror continues to be a much-sought-after objective of laser printer optical systems. The required width of the polygonal mirror surfaces in the cross scan direction has been reduced significantly by the aforementioned plano-cylindrical pre-scan lens. However, a reduction in the length of the peripheral mirror facets on the polygonal reflector is limited by beam diameter in the scan direction.
In the prior art, various designs of the pre-scan optical sub-system have been advanced to reduce the width of the laser beam directed to the polygonal scan reflector. Beam truncation by selection of the collimator lens represents one approach. The divergent beam emitted by the laser diode is truncated by the collimator lens because the clear aperture of collimator lens is smaller than the numerical aperture of the beam. Since the laser diode is aligned with its plane of widest divergence in the scan plane, the truncation effect is much larger in the scan direction. The main limiting factors in selection of the numeric aperture of a collimator lens, and thus the degree of truncation, are the cost of a collimator lens, allowable wavefront distortion due to a collimator lens, and allowable power loss. A secondary effect of truncation is the appearance of diffracted power as "lobes" associated with spots at the drum surface.
The use of aperture stops for reducing the width of the beam in the pre-scan optical system has also been proposed. This approach is also limited by allowable power loss. Finally, attempts have been made to reduce beam width at the scanning mirror facets, in the scan direction, by defocussing the collimator lens. This approach to the problem, however, results in the creation of spherical aberrations, increased wave front errors, and resulting lower print quality. Also, proper defocussing adjustment of a collimator lens requires extremely high precision, thereby making the proper adjustment very difficult to achieve in practice.
Accordingly, there is a need for improvement in the current optical systems used in devices such as laser printers, particularly a reduction in rotatable scan reflector diameter without compromising cost and print quality considerations.